<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div></div>
<script>
  function f(x) {
    let d = document.querySelector("div")
    if (isNaN(i1.value) || isNaN(i2.value)) {
      d.innerText = "输入错误";
      return;
    }
    switch (x) {
      case 1://加
        d.innerText = i1.value * 1 + i2.value * 1;
        break;
      case 2://减
        d.innerText = i1.value - i2.value;
        break;
      case 3://乘
        d.innerText = i1.value * i2.value;
        break;
      case 4://除
        d.innerText = i1.value / i2.value;
        break;
    }
  }
</script>
</body>
</html>